Radiographic imaging (or more colloquially, “X-ray” or “gamma ray” imaging) has become an important inspection tool in many scenarios, allowing non-destructive examination of regions that are otherwise inaccessible for viewing and/or measurement. The utility of this tool has been further enhanced through combination with electronic imaging. In particular, an inspected object may be illuminated with an energy source, and an image created using an array of imaging pixels. The pixels detect differing intensity levels resulting from passage of energy through the inspected object(s). The pixels convert those intensity levels to output signals, which are then converted to digital data. The pixel data can be electronically stored and processed to produce an image on a display screen or other device.
Digital radiographic imaging of certain objects presents numerous challenges. For example, when imaging adjacent objects that have significant differences in density, it is often difficult to accurately measure the separation between the objects when that separation is of the same order of magnitude as the pixel pitch of the imager. In certain applications, determining the existence of very small separations is critical. One example is the inspection of artillery shells, which include a metal casing having an interior cavity filled with an explosive. The explosive has a density similar to wax, i.e., significantly different from the density of the metal shell casing. To prevent the explosive from exploding prematurely, there must be little or no separation between the explosive and the base of the casing cavity.
A radiographic image of objects of significantly different densities will have major brightness changes in the region of transition between the objects. The human eye has difficulty distinguishing closely adjacent features of highly differing brightness. In particular, the eye tends to emphasize overall density changes while losing details resulting from the presence of a very small separation. Moreover, the finite point spread for the imager may be as large as or larger than the actual linear separation between the objects. The point spread accounts for the fact that, due to inherent limitations of imaging equipment, energy from a point source will be spatially distributed throughout a larger region of an image. As the size of a measured feature approaches the size of the point spread, the reliability of the measurement decreases.